This invention relates to the field of optical filters formed in photosensitive optical media, and more particularly, to grating formation or apodization.
Optical filters formed in photosensitive optical media by patterned exposure (e.g., interference) to actinic radiation generally have band-pass or band-stop spectral response profiles. Competing requirements for refractive index variations in the media add undesirable xe2x80x9cstructurexe2x80x9d (e.g., side lobes) to the response profiles, which is treatable by various apodization techniques.
Bragg gratings and long period gratings are examples of optical filters that can be formed in photosensitive media by patterned exposure to actinic radiation. The optical filters typically have cores that are doped with a photosensitive material, such as germanium, that enables the cores to change in refractive index in response to the exposure to actinic radiation, which is generally within the ultraviolet spectrum. The impinging radiation generally raises the refractive index of the exposed portions of the core proportional to the radiation""s intensity and the length (time) of exposure.
The required patterning, which controls both coupling strength and grating period, can be accomplished by interference or masking. Bragg gratings typically have periods less than one-half of the central wavelength of the spectral response, which is best accomplished by angularly interfering two beams of the actinic radiation. Long period gratings typically may have periods 100 to 1000 times as large and can be written by simple masking. For example, an amplitude mask can be patterned to allow spatially separated bands of light to illuminate a fiber core for forming a long period grating.
Regardless of the mode of exposure, the intensity profile of the impinging radiation translates into a similarly shaped refractive index profile of the core. For example, an impinging beam with a constant intensity profile subject to interference or masking produces uniform index modulations and a constant average index along the exposed portion of the core. The resulting spectral response, however, has large side lobes on both sides of the desired band stop. An impinging beam with a more typical Gaussian shape produces index modulations and an average refractive index that also follow the Gaussian shape. The Gaussian variation in the magnitude of the index modulations is helpful toward removing the opposite side lobes, but the accompanying change in the average refractive index produces progressive changes the effective period of the grating and typically produces side lobes on one side of the desired band stop.
Correction of the gratings to remove the undesired side lobes is sometimes referred to as xe2x80x9capodizationxe2x80x9d because it involves a xe2x80x9cshadingxe2x80x9d of grating amplitude. The goals of apodization are generally to achieve a pulse-shaped variation (e.g., Gaussian or more generally, a shape that increases to a peak and then decreases) in the magnitude of the index modulations while maintaining a constant effective period throughout the grating length. Many of the known techniques for apodizing optical gratings are expensive, time consuming, or difficult to carry out to required accuracy.
For example, U.S. Pat. No. 5,367,588 to Hill et al. teaches the mounting of a nonlinear phase mask next to the photosensitive optical filter media for exposing the media to an unevenly spaced interference pattern. The phase mask, which itself functions as a grating, divides a beam of actinic radiation having a Gaussian intensity profile into two interfering beams that form the uneven interference pattern. A varying pitch of the resulting filter grating compensates for the change in average refractive index that parallels the combined intensity profile of the illuminating beams. Such special nonlinear phase masks are expensive to manufacture and can add significant cost to the production of optical filters.
U.S. Pat. No. 5,717,799 to Robinson also proposes to correct an unwanted variation in average refractive index accompanying a desired variation in the magnitude of the index modulations by varying the grating period. Suggestions for achieving this objective include individually writing the grating elements or differentially straining portions of the grating during formation (exposure) of the grating elements. With periods as small as one-half micron for typical Bragg gratings, the writing of individual grating elements is not very practical, and differential straining of grating portions would greatly complicate manufacture and lead to potentially inconsistent results.
U.S. Pat. No. 5,309,260 to Mizrahi et al. teaches the use of successive exposures for apodizing Bragg gratings. The first exposure is performed with two interfering beams having Gaussian profiles for producing the required variation in index modulations. A second exposure with a single beam raises the average refractive index at one end of the grating for suppressing subsidiary peaks (fine structure) of the filter""s spectral response. However, variations in the average refractive index remain along grating length, which can function similar to a xe2x80x9cchirpxe2x80x9d and produce an unwanted temporal dispersion in the filtered signals.
Our invention shapes the response curves of optical filters including Bragg gratings and long period gratings by at least partially separating variations in the magnitude of index modulations from variations in the average refractive index along a optical axis of the filters. For example, a first two-beam exposure can be used to write index modulations along the optical axis of the filter, and a second two-beam exposure can be used to adjust the average refractive index of the index modulations along the optical axis.
During the first exposure, two beams of actinic radiation can be arranged to form an interference pattern of appropriate period on a photosensitive core of the intended optical filter. The two beams preferably originate from a common spatially coherent beam having an approximately sinc2 intensity profile. Axes of the two beams are inclined to each other for adjusting the fringe spacing of the interference pattern and are preferably located in a common axial plane of the filter orienting the fringes transverse to the optical axis of the filter. A crossing point of the two axes is preferably offset from the optical axis so that the two axes are relatively displaced along the optical axis.
Ordinarily, any such offset would greatly reduce fringe contrast because the interfering beams are spatially offset from each other at their point of intersection with the optical axis of the filter. However, a spatial filter can be used for shaping the common beam, which enhances the spatial coherence of the resulting interfering beams to accommodate their required misalignment. The resulting interference pattern is somewhat shorter but retains a pulse-shaped contrast profile and the same fringe spacing. The combined intensity profile of the two beams is affected most.
Offsetting peak intensities of the interfering beams along the optical axis of the filter reduces axial variation of the combined intensities of the beams within their region of overlap. The effect on the filter is to provide a more constant average refractive index within the region of overlap, while preserving a desired pulse-shaped variation in the magnitude of index modulations in the same region. The fringe contrast, which is the basis for the index modulations, decreases toward the ends of the overlap regions because of differences in intensity between the two beams. The new filter has a flattened spectral response with reduced side lobe structure.
During a second exposure, two beams are again used simultaneously in positions that are spaced apart along the optical axis of the filters. However, the spacing between the beams differs between the exposures. The first exposure forms the desired index modulations, and the second exposure cooperates with the first exposure to further level the average refractive index. The two beams can originate from the same source including the source for the interfering beams of the first exposure. However, the second exposure is not used to rewrite index modulations in the filter medium. For example, the spatial filter can be replaced by an amplitude mask that further shapes the overlapping beams but reduces spatial coherence enough to prevent fringes from forming. Alternatively, the filter medium can be dithered to average exposure intensities of the pattered illumination (i.e., xe2x80x9cwash outxe2x80x9d the fringes).
The index modulations of Bragg gratings are preferably written with either an interferometer or a phase mask, and a similar setup is preferably used for the second exposure. The two exposures are cumulative so their order can be reversed. The index modulations of long period gratings can be written with less sensitive instrumentation. For example, a rectangular function amplitude mask can be used to write the grating. However, a phase mask producing two diverging beams is preferably substituted for the amplitude mask to adjust the average refractive index at both ends of the grating and is preferably dithered for washing out any fringes.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.